The present invention relates to systems for controlling electric motors and, more particularly, to systems for deriving accurate input parameters for use by electric motor controllers.
All electric motors are characterized by a counter EMF which opposes the terminal voltage during the operation of the motor. For example, in accordance with the dq axis model of an AC induction motor the counter EMF components or speed voltages w.sub.e .lambda..sub.qe and w.sub.e .lambda..sub.de along the d and q axes in the synchronous frame of reference are related to other voltage parameters as shown in equations 1 and 2 below: EQU V.sub.qe =r.sub.s i.sub.qe +w.sub.e .lambda..sub.de +p.lambda..sub.qe eq. ( 1) EQU V.sub.de =r.sub.s i.sub.de -w.sub.e .lambda..sub.qe +p.lambda..sub.de eq. ( 2)
where:
V.sub.qe, V.sub.de =terminal voltage components PA1 r.sub.s =stator resistance PA1 i.sub.qe, i.sub.de =terminal current components PA1 w.sub.e =field frequency PA1 .lambda..sub.de, .lambda..sub.qe =flux linkage components PA1 p=derivative operator
Equations 1 and 2 can be simplified for steady state conditions and solved for the speed voltages as shown in equations 3 and 4 below: EQU w.sub.e .lambda..sub.de =V.sub.qe -r.sub.s i.sub.qe eq. ( 3) EQU w.sub.e .lambda..sub.qe =r.sub.s i.sub.de -V.sub.de eq. ( 4)
Deriving the counter EMF components is then simply a matter of measuring the terminal voltages, transforming into the synchronous frame of reference and differencing these voltages with the voltage drops r.sub.s i.sub.qe and r.sub.s i.sub.de. However, while the currents i.sub.qe and i.sub.de can be sensed or formulated as command quantities, the stator resistance r.sub.s is difficult to ascertain with accuracy under most operating conditions. Fixed estimates of stator resistance r.sub.s are quickly rendered inaccurate by conditions inherent in motor operation which cause heating of the stator winding. Alternately, direct measurements may be made of stator resistance r.sub.s by the injection of signals into the stator winding. However, such signal injections inevitably result in disturbance of the motor system which is unacceptable under most operating conditions. Therefore, while the speed voltages remain important quantities to be determined in many motor control systems, adequate techniques have not been available for deriving these quantities with accuracy during motor operation.
It is therefore an object of the present invention to provide a system for determining counter EMF parameters during motor operations with a high degree of accuracy.
It is yet another object of the present invention to provide a system for determining counter EMF or speed voltage parameters during motor operations which is not invasive and provides no disturbance of the motor system.